Fabian Bate, who’s battling leukemia, posted the letter on his blog. The note was written on Oct. 13 on St. James’s Palace letterhead and signed, simply, “Catherine.”

“Despite the enormously demanding course of treatment you are undergoing, I was so touched by your strength of character, and delighted to hear the news that one of your big sisters will be able to donate bone marrow to you later this year,” she wrote. "I will keep my fingers crossed that your health goes from strength to strength over the months ahead.

“This must be a troubling time for you, your parents and your sisters, but I know I left The Royal Marsden assured by how incredibly talented, kind and clever the team at the hospital are. Combined with your belief and positive energy, you couldn’t be in better hands.”

“Keep up the good work with the blog,” she concluded, “and in the meantime I will keep you and your family in my thoughts and prayers.”